Bi-directional optical communication transmits and receives simultaneously optical signals in both directions of a single optical fiber. In each direction, the optical wavelength in use is different, which provides two independent path of communication even if the path is single. Consequently, at the end of an optical fiber, the optical signal of outgoing is to be coupled into the reverse path of incoming signal and two conversions of signals, one from electrical to optical for transmission and the other from optical to electrical for reception, take place. More general form of bi-directional optical communication uses more wavelengths than the two wavelengths in each direction. The wavelengths in each direction then need to be splitted or combined at the termination of optical fiber; and the signals of each optical wavelength is to be converted either from electrical to optical or from optical to electrical.
An example of such a bi-directional device is a BiDi (bi-directional; simply written as BiDi) diplexer for use in the FTTH (Fiber-to-the-Home) network of optical communication, which uses 1.31 μm wavelength for transmission from each subscriber to the central office (upstream signal) and 1.55 μm wavelength for reception at each subscriber (downstream signal) in the network. Here, the ‘diplexer’ implies two wavelength device. Therefore, the termination of optical fiber at the subscriber is to be connected by the optical device of 1.31 μm transmission together with 1.55 μm reception into a single optical fiber to the central office.
On the other hand, an extra wavelength, in addition to the two wavelength of the diplexer, is used to send a signal such as CATV (Cable TV) to each subscriber, which is referred to a BiDi-triplexer or, simply, a triplexer. In this case, downstream digital signal uses 1.49 μm wavelength; upstream digital signal uses 131 μm wavelength; and downstream analog signal uses 1.55 μm wavelength. Furthermore, the fourth wavelength of 1.61 μm for further expansion is considered by some service providers.
As the number of wavelength channel in use increases in this manner, the number of optical components in use increases and the optical alignments in the assembly of the components becomes a critical issue in the production of device. For example, the core of a common optical fiber is about 10 μm and the tolerance of less than a micron in the optical alignment is necessary in the procedure of optical alignment fixing an optical fiber and the optical devices in use (transmitter or receiver; normally laser diode or photodiode).
In case of BiDi triplexer, for example, the optical alignment using the former technology has about 50 freedoms to fix in the alignment; and consequently one assembly of the device takes more than 10 minutes resulting in a serious bottleneck of production.
FIG. 1 shows the structure of a conventional BiDi-triplexer using former optical-filter technology. Such procedure is illustrated in the “Development of 3 TO-Triplexer Optical Sub-Assembly” (Photonics Conference 2004, paper number T1A2, Optical Society of Korea, Jungwan Park, et. al., Samsung Electronics).
Referring to FIG. 1, BiDi triplexer is composed of three TO-CAN's (Transistor Outline Can; 21, 22, 23) of one laser-diode and two photodiodes and four optical thin film filters 26,27,28,29. Transmitter TO-CAN 23 is pre-assembled with laser diode 23c, lens 23a, and monitor photodiode 23b; and receiver TO-CAN's 21,22 are pre-assembled each with photodiodes 21b,22b and lenses 21a,22a. The lenses 21a,22a,23a in front of each TO-CAN 21,22,23 and the lens 24 terminating the optical fiber 25 ensure collimated passage of light beam between these components 21,22,23,25.
Explaining the operation of the device, the receiving signals of 1.55 μm and 1.49 μm wavelength coming from the optical fiber 25 are divided by the film-filter 26,27,28,29 in free space, then reaching the receiver photodiodes 21,22. The transmitting signal of 1.31 μm wavelength coming from the laser diode 23 passes through the two consecutive filters 28,26 then reaching the optical fiber 25. In front of two photo-diodes 21,22, the blocking filter of 1.55 μm or 1.49 μm wavelength 27,29 which cuts off the other wavelength than the one of the corresponding photodiode is arranged.
The triplexer using the conventional technology introduces heavy task in the optical alignment. To solve the problem, a method of using a planar optical waveguide instead of using many individual components was introduced. According to this method, the optical alignments between components can be minimized; and components such as film-filters, transmitter, and receivers, can be assembled on a single chip. This method reduces significantly the number of freedom to fix in the optical alignment because optical waveguide can connect signals between the constituent components.
FIG. 2 shows the structure of triplexer using a conventional technology of optical waveguide, which is released in Japan laid-open JP 1998142459, (‘Waveguide type optical module’, Kyocera corp). Referring to the FIG. 2, the light signal launched through the input port 14 from the optical fiber 1 passes through the optical waveguide 18 entering the film-filters 7,8 in the groove 9, where the signals combine or split the wavelengths by passing or reflecting according to the wavelengths. The signal of 1.3 μm wavelength coming from the optical transmitter 5 is reflected by the film-filter 8 reaching the optical fiber 1. The signals of 1.49 μm and 1.55 μm wavelength come in from the optical fiber 1. The 1.49 μm signal passes two consecutive film-filters 7,8 reaching the receiver photodiode 4; and the 1.55 μm signal is reflected by the film-filters 7 reaching the optical fiber 2. In the termination of the optical fiber 2, a receiver photodiode may be attached.
In the structure of the triplexer, fragile film-filter 7,8 with the thickness of 0.1˜0.01 mm is to be prepared together with the matching groove 9 for insertion of the filter 7,8; and a delicate procedure of film insertion into the groove 9 is to be performed. Such an optical filter 7,8 is usually prepared by a coating on a glass substrate, followed by separating it from the substrate then cutting it into appropriate size. Such procedure is to be repeated one by one. This involves still a significant problem in production even though there is some improvement compared to the former methods of using the filter in free space as in the FIG. 1.
While the patent “OPTICAL COMPONENT FOR FREE-SPACE OPTICAL PROPAGATION BETWEEN WAVEGUIDES” (U.S. Pat. No. 7,031,575 B2, Xponent Photonics Inc.) also releases a module similar to the method in the FIG. 2, but this method still involves similar problem of inserting thin films into grooves encountered previously in the production of the devices.